Sound attenuating device



March 30, 1937. R. B. BOURNE 2,075,263

SOUND ATTENUATING DEVICE Filed Oct. 19', 1951 2 Sheets-Sheef 1 5.1. 2/vg Bf as U12;

H mm m fig. ,3

, @2- L v. l LE;

fig .4-

w a; a;

l U U I INVENTOR.

Ram/v0 3300M:

ATTORNEYS.

March 30, 1937. R- B. ou N 2,075,263

SOUND ATTENUATING DEVI CE Filed Oct. 19, 1931 2 Sheets-Sheet 2 la L I RgVBENTOR.

am no mm: BY *Q W440 A TTORNEY S.

Patented Mar. 30, 1937 UNITED STATES PATENT OFFICE SOUND ATTENUA'I'INGDEVICE Roland B. Bourne, Hartford, Conn, assignor to The Maxim SilencerCompany, Hartford, Conn., a corporation of Connecticut ApplicationOctober 19, 1931, Serial No. 569,111

7Claims.

10 or less continuously from the lower to the upper limit of audibility.

One object of the invention is to provide a com mercially practicaldevice for attenuating sound waves over -a wide range of frequencies. Afur-.

ther object is to provide a silencing device having a wide attenuationrange which will impose practically no back pressure upon the system. Afurther object is to provide a device of this general character whichwill be compact in its over- 0 all dimensions. A further object is toprovide a device of this general character in which the necessity forclosely and accurately spaced walls is eliminated, thereby resulting ina decrease in the cost of manufacture. A further object is to provide adevice of this general character which is inexpensive in itsconstruction. I

For the purpose of this specification, the sound spectrum is dividedroughly into two groups: sounds of high frequency, which are mostefflciently attenuated by the use of dissipative materials; and soundsof low frequency, which are most easily attenuated by the use ofreactive side branches.

Acoustic'friction devices for attenuating high frequencies may be placedin series with reactive devices for attenuating low frequencies. Such acombination, while very effective in sound attenuation, is generallycommercially unwieldy and expensive. It has been found that satisfac- 40tory attenuation of both high and lowfrequencies may be accomplished bycombining the separate means therefor in one device, simple ofconstruction and cheap to build.

If enough acoustically absorbent material,

suitably disposed, is used, it is possible to secure by this means alonesatisfactory attenuation of evenvery low frequency sound waves. Byemploying reactive side branches in combination with acoustic absorbentmaterial, the length of the main conduit and consequently the backpressure is greatly reduced and makes possible commercial embodimentspossessing the further advantages of small size, light weight and a highdegree of sound attenuation.

, For a further explanation and understanding of the devices, referenceis now made to the draw-.

ings, in which:

Figs. 1 to 5, inclusive, are diagrammatic views illustrating theacoustic principles of different forms of the invention;

Figs. 6, 8, 10 and 12 are central longitudinal sections throughdifferent forms of the invention; and

Figs. '7, 9 and 11 are sections on lines 'l-1, 9-9, and. ll-l| of Figs.6, 8 and 10, respectively.

side branches on a main acoustic channel for attenuating sound waves ofcertain frequencies passing therethrough is old in the art. A'singlesuch side branch may be expected to offer attenuation to thosefrequencies for which the side branch is resonant,--that is, since aclosed pipe resonates to frequencies for which it is an odd number ofquarter wave lengths long, these frequencies and those immediately aboveand below it will suffer attenuation. The sharpness of the resonancecurve depends upon the relation of the diameter of the side branch tubeto that of the main conduit, as well as on the dissipation present inthe resonator. The attenuation falls off on either side of the resonantfrequency. Since the problem presented is one of attenuating acontinuous band of frequencies, it becomes pos sible to accomplish thisby providing a number of closed tubes of different lengths, suitablydisposed along the main sound conduit, these lengths being so chosenthat the attenuation bands for the various side branches will over-lapeach other.

Since I am not interested in obtaining pass bands but rather acontinuous attenuation band, it is not only not necessary to obtain amatching of impedances between sections oi a filter but desirable tosecure mismatching, since a certain amount of, attenuation is therebypossible. This attenuation is due to reflection losses. One way ofobtaining this mismatching is by making the distance between sidebranches, along the main channel, of unequal length. Referringparticularly to Fig. 1, the closed tubular side branches 20 and 2i aredisposed laterally along the length of and with respect to the mainchannel 22. For instance, the side branch 2| is, in this case,arbitrarily taken as three-quarters of the length of side branch 20, andso on, In practice the length of the longest side branch is determinedby the lowest frequency it is desired to attenuate. In some cases, wherethe frequencies to be attenuated are very low, the length of the sidebranch is necessarily excessive. It has been found that The use ofclosed conduits or tubes as acoustic no appreciable diminution ofattenuation is suffered by bending the tubular member back upon itself.This feature permits of a more compact arrangement of tubes.

5 In Fig. 2, the main channel 24 has disposed along its length the .sidebranches 25 and 25. The side branches 25 are equal in length and are atopposite ends of the channel. Between these are disposed the sidebranches 26, also equal in length but shorter than the side branches 25.In Fig. 3, the side branches 21-, 28, 2E and 30 are not only of aprogressively shorter length but also are disposed along the conduit 30at different distances apart. In this filter is also shown a terminalimpedance in the form of a straight tail pipe 32. In Fig. 4, we have themain channel 33 with a plurality of unequal. closed tubular sidebranches disposed along its length but in such a manner that the shorterside branches act not only as efficient attenuators forthose frequenciesfor which they are resonant, butalso act as acoustic loading on the mainchannel for frequencies much lower than those for which they areresonant. For

instance, the side branch 34, placed between the side branches and 35,acts to load the main channel 33 and thus simulates a greater lengthbetween the junction points of said side branches 35 and 35. Thisloading, in reality, effects a rotation of phase. Similarly side branch31 loads the main channel 33 between the side branches 36 and 38.

The principle of phase rotation by loading may be also applied to sidebranches. In Fig. 5, 35 the main channel 39 is furnished with tubularside branches 40, 4| and 42, and also with intermediate short branches43 and 44 which act to load the main channel 39. Side branch 40, itself,is acoustically lengthened by the presence of the sub-side branch tubes45 and 43. Similarly, side branch 4| is acoustically lengthened by theloading action of 41 and 48. Side branch 42 has been shown as providedwith but one subside branch 48. This feature makes possible the compactassembly of various side branch tubes for commercial embodiments of theinvention. This acoustic loading may be also accomplished by the use ofsuitable volumetric side branches.

The upper limit of attenuation attainable is represented by the shortestpracticable side branch. To obtain attenuation for higher frequencies,use is made of the sound absorptive properties of certain materials. Theuse of soft porous materials for absorbing sound is well known. Ductsand channels lined with felt, for

instance, will absorb sound waves of medium to high frequencies passingtherethrough. The amount of attenuation obtainable depends upon theamount of material used, its disposition, its quality, and the frequencyof the sound to be attenuated. Most such materials attenuate mostefllciently at frequencies of the order of 1500 cycles per second. Sinceattenuation by absorption is not particularly selective, this offers a65 means for preventing transmission through conduits of a very wideband of frequencies which lie above the point of feasible attenuation byreactive devices.

In order to utilize both types of attenuatiorTih one device, severalexpedients may be employed. Fig. 6 shows one embodiment of such adevice, comprising a shell 50 fitted with headers 5| and necks 52.Extending between the two necks 52 and supported thereby is a perforatedmetal conduit 53 which forms the main channel 54, Disposed along thelength of said conduit 53, in this case with their respective couplingpoints at equal distances apart, are the four side branches 55, 56, 51,and 58. These side branches correspond acoustically to those shown inFig. 2. They are disposed with their axes parallel to that of the mainchannel 54 and are connected thereto by means of necks 59. It will beseen that this disposition of the side branch tubes leaves considerablespace between the shell 50 and the central tube 53. This space is filledwith sound absorbing, heat resisting material 50 which provides for theattenuation of the high fre- 'quencies. It will be noted that thefourside branches are disposed angularly around the main tube 53, thusproviding a compact assembly. In order that the sound waves may haveample opportunity to penetrate all the sound absorbent material, theside branch tubes 55, 56,. 51 and 58 are so disposed that the sum of theshortest peripheral distances between adjacent tubes is at least equalto the perimeter of the main conducting tube 53. This feature applies toall devices of this character. It is found in practice that the lowfrequency attenuation may be affected to a slight degree by using aperforated main conduit rather than a solid one. Careful measurements ofa multiple sidebranch silencing device with and without dissipation inthe main channel between the points of coupling of the sidebranches showthat the attenuation due to the dissipative material is simply additiveover a considerable portion of the sound spectrum, becoming greater ineffect, as would be expected, with higher frequencies. At points whereseries resonance in the main channel between the junction points of thesidebranches occurs, however, the effect of the addition of dissipativematerial to that portion of the main channel is strongly marked,destroying largely the adverse effect of the resonance in the mainchannel in amplifying frequencies which would otherwise be attenuated bythe sidebranches. At these points in the attenuation curve the increasein attenuation due to the dissipative material is much greater thanwould be due to the sound absorptive qualities of the material itself,since dips in the attenuation curve due to series resonance may bechanged to actual peaks. In acoustic devices having a plurality ofsidebranches those peak attenuations which are due to the interactionbetween the sidebranches may be actually reduced by the addition ofdissipation in the main channel, but this is not disadvantageousas thesepeaks are so much higher than the general level of attenuation of thedevice as a whole that their reduction does not cause undesirablepassage of sound. In general it may be said'that in addition to thegeneral raising of the attenuation curve by the use of dissipation inthe main channel between the sidebranches, which as has been statedabove is enhanced in the higher frequencies, there is a valuablesmoothing out of the peaks and valleys of the curve, with a greater gainin the valleys than is lost in the peaks. Careful designing, withrespect to the kind of sound absorbing material used and number and sizeof perforations in the main conducting channel, makes it possible toobtain, in combination with the reactive side branches, satisfactoryattenuation over as wide a frequency band as is desired. The presence ofthe sound absorbing material 60 which, is

packed around and between the various tubular members effectivelyprevents shell noise and amazes also prevents metallic ringing of theinterior parts of the silencer. It also confines most of the heat ofexhaust gases to the interior of the silencer, an important feature inconnection with interior installations.

Fig. 8 shows an embodiment of the invention wherein the tubular sidebranches are of progressively shorter lengths and are spaced atprogressively shorter distances apart along the main 10 channel 6i. Thedevice is thus acoustically similar to that shown in Fig. 1. The lengthsof the side branches 62, 63, 64 and 65 are so chosen as to cover (withthe aid of harmonic resonances) the desired frequency band. A packing 66of sound absorbing material is also employed in this case,surroundingthe central perforated tube Bl.

In some cases, it is desirable to use. instead of a tubular side branch,one of the volumetric type. This would be indicated in the case of anair compressor intake silencer where the surge or thump is of very lowfrequency and where it is desirable to provide a certain reservoir ofair upon whichthe compressor may draw. Such a device is shown in Fig.10. The central tube 10 which forms the main channel is supported byheads 1| fastened to an outer shell 12. Also attached to each of theheads 'II are shells l3 and 14 provided with heads 15 apertured toreceive the central tube I0 and spaced apart so as to provide a spaceI6. This space communicates with the single annular volumetric sidebranch 11 which is formed jointly by the outer shell 14 and theintermediate shells 13, ll. The tube 10 is perforated so that the soundwaves passing through it may contact with the sound absorbing material18 packed between the tube and the intermediate shells 13, M.

Fig. 12 shows an embodiment of the invention in which the main channel80 has disposed along its length a plurality of closed volumetric sidechambers 8| formed by an outer shell 82,v the headers 83, andintermediate shells 84. In this case, the side branches are ofprogressively smaller size and communicate with said channel 80 throughopenings 85 therein. In addition, the channel 80 offers attenuation forhigh frequencies by the presence of the sound absorbing material 86disposed more or less continuously along its length. This material ispacked between the perforated channel tube 80 and the shells N withtheir individual headers.

It is obvious that many combinations usin non-dissipative anddissipative side branches may be conceived and reduced to practicalworking embodiments. Without enumerating a large number of sucharrangements, it is clear that the scope of the invention embraces manycombinations not shown in detail, and I therefore do not limit myself tothose exact embodiments depicted but claim additional and obviousembodiments suggestible to those skilled in the art.

What I claim is:

1. A silencer comprising an acoustically dissipative main conductingchannel and a plurality of non-dissipative acoustic side branchesacoustically coupled thereto at intervals along the.

length thereof.

2. A silencer comprising a casing, a sieve-like main conducting channelextending therethrough, acoustically dissipative material between saidcasing and said channel, and a plurality of non-dissipative acousticside branches connected to said main conductive channel at intervalsalong the length thereof.

3. A silencer comprising a main conducting channel and an alternateseries of dissipative and non-dissipative acoustic elements laterallydisposed with respect thereto and acoustically coupled thereto atintervals along the length thereof.

4. A silencer comprising a cylinder-like casing, headers in said casing,a sieve-like main conducting channel extending between said headers,acoustic absorbent material between said casing and said main conductingchannel, and a plurality of closed tubular side branches acousticallycoupled to said main conducting channel at intervals along the lengththereof, said closed tubular side branches being disposed with theiraxes substantially parallel to the axis of the main ROLAND B. BOURNE.

